Harriet Harman, the Labour deputy leader, today unveiled plans for her party to become the voice of southern indignation with the government in the May council elections.

In an interview with the Guardian she said Labour needed to start the process of replacing the Liberal Democrats as the anti-Tory voice.

"It is the first test of this government. There are large areas where people have voted Lib Dem as the progressive choice but perhaps now they are unlikely to want to vote with a party that has broken so many of their promises," she said.

"One of our key aims is to ensure we are fielding candidates across the country – even in those areas where we may not have previously done very well – to ensure people have the option of voting Labour. There is a great deal of indignation and we will be people's voice in these tough times."

Party officials have been looking at whether it can switch limited organisational and financial resources from its northern heartlands in the elections as part of an effort to build a stronger bridgehead in the south.

On 5 May, 80% of England's councils face elections, with 9,330 seats being contested. There are also elections for the Scottish parliament and Welsh assembly and the referendum on the alternative vote for Westminster elections. Labour holds only 460 of the 4,800 seats being contested in the southern, eastern, and south-west regions of England – in the recent past Tory and Lib Dem battlegrounds from which Labour has been excluded.

By contrast there are only 900 seats up for grabs in the north-west and Labour should be able to push back Liberal Democrats in the northern cities. The greater challenge for the party is to make inroads in areas that it long ago lost influence.

By one estimate nearly half of the councils in the south-east do not have a single Labour councillor.

Harman said: "We want to show that we are standing on the side of people in the south, the south-west and the east who are as every bit as indignant as people in the north and north-west about tuition fees, the future of the NHS, spending cuts and police numbers.

The Labour strategy, its first campaign in opposition for 14 years, is simple. "It is going to be an opportunity to send a message to the government and for us to say we will be your voice in tough times."

Most of the seats were last contested in 2007 while Gordon Brown was succeeding Tony Blair, with the Tories winning a 40% share of the vote, and the Liberal Democrats and Labour getting around 27% each. Some 5,105 Tory-held seats are being contested, 1,870 Liberal Democrat and only 1,570 Labour.

"There is a momentum. Every week another group of people has got a personal reason to feel let down with this government," Harman said.

But the party is so far behind in some parts of the south – and Liberal Democrat councillors are famously good at digging themselves in – that Labour thinkers such as Jon Cruddas are arguing that culturally and linguistically the party is almost no longer part of England.

Pollsters such as Deborah Mattinson found in focus groups in swing seats such as Harlow, Essex, that Labour's reputation for economic competence was "in tatters".

Equally there is some evidence from the Oldham East byelection that coalition supporters, or at least some Conservative voters, are willing to vote tactically to keep Labour out.

A rash of unpopular strikes, some designed to coincide with the royal wedding, would hardly help Labour's appeal.

Harman said: "We are not predicting how people are going to react to the very different politics to the general election. People do not move like a pollster's herd and people are affected very personally by different things.

"So far as the Liberal Democrats are concerned, we are not going to do any too-clever-by-half differentiating with the Conservatives.

"The simple fact is that in many places they have ruled themselves out by breaking promises and by posing as a progressive party at the general election."